1. Field of the Invention
This invention relates to an asymmetrical lens adapted to optically couple optical waveguides having different optical characteristics with a low optical loss level.
2. Prior Art
As the single mode optical fiber gains popularity in the field of optical telecommunications and other optical technologies, there has been an ever increasing demand for optically coupling an optical fiber and other optical waveguides having a configuration different from that of the optical fiber such as semiconductor laser devices and/or thin film optical waveguides particularly in the field of advanced optical telecommunications network.
Any two optical waveguides can be mutually connected by means of a rotatably symmetric lens such as a spherical lens or a nonspherical lens as long as they show an optical intensity distribution of waveguide mode which is rotatably symmetric relative to the optical axis that operates as the axis of symmetry on a plane perpendicular to the optical axis.
Such optical connection can be realized with a low optical loss level because the optical elements involved in the optical connection have an identical waveguide mode.
On the contrary, optical connection of any following combinations of optical waveguides has not been technically feasible without producing a significant optical loss:
(1) an optical waveguide operational with a waveguide mode having the above defined symmetry, e.g., an optical waveguide showing a rotatably symmetric circular beam pattern (axis of symmetry: optical axis), and an optical waveguide operational with a waveguide mode having no such symmetry, e.g., an optical waveguide showing an elliptic beam pattern which is not rotatably symmetric, PA1 (2) two optical waveguides showing respective elliptic beam patterns that are not similar to each other in terms of mode configuration, and PA1 (3) two optical waveguides showing astigmatism along vertical and horizontal directions of radiation pattern. PA1 Paper 3: "Introduction of Optical Electronics" A. Yariv; translated by Kunio Tada and Takeshi Kamiya, 1988, Maruzen K. K. PA1 Paper 4: "The Basis and Application of Optical Coupling System for Optical Devices" Kenji Kohno, 1991, Gendaikogakusha. EQU m=w2/w1 [1] PA1 (f.perp.1) and (f.perp.2) respectively represent the distances between the two principal surface planes of the lens as projected on one of the planes and the image forming spot (beam waist), being a symbol indicating a vertical direction, PA1 (s.perp.1) and (s.perp.2) respectively represent work distances corresponding to the above distances, PA1 (f.parallel.1) and (f.parallel.2) respectively represent the distances between the two principal surface planes of the lens as projected on the other planes and the image forming spot (beam waist), being a symbol indicating a horizontal direction, PA1 (s.parallel.1) and (s.parallel.2) respectively represent work distances corresponding to the above distances, PA1 (w.perp.1) and (w.perp.2) respectively represent the major and minor axes of the image forming spots having elliptic and/or circular forms on the light emitting terminals of the optical waveguides connected by way of the lens for matching of their respective waveguide modes, PA1 (d1) and (d2) respectively represents the degrees of astigmatism of the optical waveguides connected by way of the lens, and
Currently, a cylindrical lens, a prism or another rotatably symmetric lens is used in such a way that the lens is inclined by a given angle relative to the optical axis in order to accommodate itself to the above described combinations of optical waveguides. However, such an arrangement is inevitably accompanied by a significant optical loss.
Technological proposals using a rotatably asymmetric lens in an attempt to minimize the optical loss in coupling two optical waveguides that fall in one of the above defined combinations are found in published papers including Japanese Patent Laid-open Publications Nos. 62-191803 (hereinafter referred to as Paper No. 1) and 3-172801 (hereinafter referred to as Paper No. 2). However, none of these known techniques has succeeded in reducing the optical loss in optical connection.
The reason for this lies in the fact that none of those proposals does not provide a theory for designing a lens to be used for optically coupling two optical waveguides as defined in any of the above combinations (1) through (3).
Hence, no lenses, lens systems nor lens devices have so far been designed to minimize the optical loss in coupling two optical waveguides that fall in any of the above defined combinations (1) through (3) and meet the demand of constructing an advanced optical telecommunications network.
Now, some of the currently available theories for designing a lens and the status quo of the technology for producing a lens will be briefly discussed.
Optical waveguides used in advanced optical telecommunications networks are normally designed to operate in a transversal direction only with a fundamental waveguide mode for optical transmission as in the case of single mode optical fibers and buried type semiconductor laser devices.
In optical transmission using such optical waveguides, the waveguide mode is defined as a function of the first order Gaussian beam, which provides a significantly favorable approximation as described in Paper 3 as listed below.
In an analysis using the first order Gaussian beams, equation [1] below provides a necessary and sufficient condition for a lens system of magnifying power m in order to establish an ideal connection between two optical waveguides that are similar to each other and have respective spot sizes wl and w2 that are different from each other (provided that the numerical aperture NA of the lens system does not impose any restrictions on incident light). This is described in detail in Paper 4, listed below.
With a ray tracing method, unlike a method utilizing Gaussian beams, the numerical aperture NA of each optical waveguide to be coupled in an optical system provides a condition to be met for optical connection along with the above described spot size.
However, such a ray tracing method cannot effectively be used for optical connection of optical waveguides, because it neglects parameters relating to the phase of light passing through each optical waveguide.
Incidentally, with a method utilizing Gaussian beams, the numerical aperture NA of each optical waveguide in an optical system is automatically, complementarily and unequivocally determined by the spot size of the optical waveguide because of constraints imposed on the phase of light.
In order to converge Gaussian beams to a small spot, the numerical aperture of an optical waveguide needs to have a relatively large value. This explains why the radiation angle NA of beams of light emitted from an optical waveguide having a large spot size is small.
In view of these, it will be apparent that a technique disclosed in the Paper 1 cannot theoretically optimize the optical connection between two optical waveguides because it deals with NA and image magnifying power independently in designing a lens system for the optical waveguides.
In other words, when two optical waveguides having different NA values are connected by way of a lens system, the spot formed by beams of light that are converged by the lens system will inevitably become larger on the part of the optical waveguide having a smaller NA than on the part of the other optical waveguide having a larger NA.
Thus, with a technique of the Paper 1, any two optical waveguides cannot be optimally connected with each other by means of a lens system.
While the Paper 2 proposes a technique for matching of two optical waveguides by means of a lens system having an optimized image magnifying power, it fails to take the spot size and location into consideration for optical waveguides.
In other words, with a technique proposed in the Paper 2, matching of image magnifying powers of a lens system for two planes (in vertical and horizontal directions) that are parallel to the optical axis of the lens system and perpendicular to each other is handled independently for each plane.
While the Paper 2 seems to reveal an intention to meet the condition of equation [1] if said two planes in vertical and horizontal directions are taken separately, it is not possible by any means to achieve an optimized status of connection simultaneously for both vertical and horizontal directions because the optimum spot (image forming spot) for a vertical direction is displaced from its counterpart for a horizontal direction.
Thus, a technique of the Paper 2 cannot optimally couple any two optical waveguides by way of a lens system.
Additionally, the fact the Paper 2 does not propose any technique for compensating astigmatism also evidences that it is not appropriate for providing optimum conditions in optically coupling optical waveguides.